Recently, NMT has improved the results of the phrased-based systems [1]}.
Besides these improvements in NMT, some problems still remain.
One problem is the high computational cost of the target word probability due to the softmax that requires to normalize all the output values, see Equation REF :
\( p_i = e^{o_i} / \sum _{r=1}^N e^{o_r} ~\mathrm {for}~ i \in \lbrace 1, \dots , N\rbrace \) 
